RxJS Interview Questions and Answers for 10 years experience

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100 RxJS Interview Questions and Answers

  1. What is RxJS and what are its core concepts?

    • Answer: RxJS (Reactive Extensions for JavaScript) is a library for composing asynchronous and event-based programs by using observable sequences. Its core concepts include Observables (streams of data), Operators (functions that transform Observables), Subjects (special Observables that allow both pushing and receiving data), Schedulers (control when operations execute), and Observers (functions that react to Observable emissions).

  2. Explain the difference between Observables, Promises, and Events.

    • Answer: Promises represent a single future value, while Observables represent a stream of multiple values over time. Events are typically tied to specific DOM elements or user interactions, whereas Observables can represent a broader range of asynchronous data streams. Observables offer more powerful composition and cancellation capabilities than Promises.

  3. Describe the different types of Observables and when you would use each.

    • Answer: There are various ways to create Observables: `of`, `from`, `fromEvent`, `interval`, `timer`, `create`. `of` emits a defined set of values then completes. `from` converts arrays, Promises, Iterables into Observables. `fromEvent` creates Observables from DOM events. `interval` emits values at fixed intervals. `timer` emits a value after a delay and optionally repeatedly. `create` allows fully custom Observable creation.

  4. What are RxJS operators and how do they work? Give examples of common operators.

    • Answer: RxJS operators are functions that transform Observables. They allow chaining operations to manipulate data streams. Common operators include `map` (transforms emitted values), `filter` (filters emitted values based on a condition), `reduce` (aggregates emitted values), `mergeMap` (handles multiple inner Observables), `concatMap` (executes inner Observables sequentially), `switchMap` (cancels previous inner Observables when a new one starts), `debounceTime` (ignores emissions if another one comes within a time frame), `catchError` (handles errors).

  5. Explain the concept of hot and cold Observables.

    • Answer: A cold Observable only begins emitting values when a subscriber subscribes. A hot Observable emits values independently of subscribers; new subscribers only receive the stream from the point of subscription.

  6. How do you handle errors in RxJS?

    • Answer: Errors are handled using the `catchError` operator. This operator intercepts errors from the Observable stream and allows you to either handle the error directly or provide an alternative Observable.

  7. What are Subjects in RxJS and what are their different types?

    • Answer: Subjects are special types of Observables that allow you to push values into the stream. Types include `Subject`, `BehaviorSubject` (emits the last emitted value to new subscribers), `ReplaySubject` (emits a buffer of the last N values), and `AsyncSubject` (only emits the last value when the source completes).

  8. Explain the use of Schedulers in RxJS.

    • Answer: Schedulers control when operations are executed. They can be used to control the timing of emissions, especially useful for performance optimization or to avoid blocking the main thread. Examples include `async`, `queue`, `animationFrame`, and custom schedulers.

  9. How do you unsubscribe from an Observable to prevent memory leaks?

    • Answer: Unsubscribe using the subscription's `unsubscribe()` method. In Angular, using the `async` pipe automatically handles unsubscribing when the component is destroyed. For other scenarios, ensure you unsubscribe in `ngOnDestroy` or other appropriate cleanup methods.

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